Ratio detector circuit for frequencymodulated oscillations



June 7, 1955 A. H. J. N. VAN DIJKUM I 2,710,350

RATIO DETECTOR CIRCUIT FOR FREQUENCY-MODULATED OSCILLAT IONS Filed Sept.23, 1953 INVENTOR ADELBERTUS HERMANUS JACOBUS NIEVEEN VAN DUKUM GENTRATES DETECTOR CIRCUIT FOR FREQUENCY- MODULATED OSCILLATIONS AdelbertusHermanus Jacobus Nieveen Van Dijkum, Eindhoven, Netherlands, assignor toHartford National Bank and Trust Company, Hartford, Conn., as trusteeApplication September 23, 1953, Serial No. 381,870 Claims priority,application Netherlands October 13, 1952 1 Claim. (Cl. 250-27) Thisinvention relates to a circuit for demodulating frequency-modulatedoscillations by means of a ratiodetector of the type comprising twocoupled circuits, two rectifiers and an RC-filter which ofierssubstantially a short-circuit to the modulation frequencies. Itsparticular object is to improve in a simple manner the characteristiccurve of a frequency-detecting circuit of the said kind on reception oflarger input amplitudes.

According to the invention the said RC-filter comprises a VDR-resistor.

The term VDR-resistor is to be understood to mean a resistor body, theresistance of which greatly decreases with increasing voltageirrespective of the direction of the voltage impressed. The materialused to constitute such a resistor is frequently silicon carbide.

In order that the invention may be clearly understood and readilycarried into eifect it will now be described with reference to theaccompanying drawing, in which two embodiments are given by way ofexample.

Referring to Fig. 1, the frequency-modulated input oscillations producedacross the anode circuit of a tube 1 and required to be demodulated arefed via two circuits 2 and 3, which are about critically coupled andtuned to the input frequency, to two rectifiers 4 and 5 opposite inconductive direction, the output of which includes an RC-filtercomprising a resistor 6 and a capacitor 7 and ofiering substantially noresistance to the modulation frequencies. Via a coil 11 tightly coupledto the inductance of the primary circuit 2 the midpoint of the secondarycircuit 3 is connected to a detector-output filter 8 so as to permit thedemodulated oscillations to be abstracted from an ouput terminal 9.

As a rule, a ratio detector exhibits full limiting only for a givenvalue of the mean input amplitude.

According to the invention a VDR-resistor 10 is connected in parallelwith the RC-filter 6, 7. Such a resistor has a characteristic curvewhich may be reproduced approximately by the formula V=Ci, where Vdesignates the voltage across the resistor, i the current passingthrough the resistor and C and p designate constants. The values C arewidely differing for the various types in which the said resistors areto be had. The value of B ranges about between 0.2 and 0.35.

ited States Patent 0 ice If the input voltage across the circuit 2 isstill so small that at the corresponding voltage at the smoothing filter6, 7 the resistance of the VDR-resistor 10 is high as compared with thatof the resistor 6, the influence of the VDR-resistor 10 is substantiallynil. However, if with increasing input signal the voltage at thesmoothing filter 6, 7 is such that the resistance of the VDR-Resistor 10is of the same order as that of the resistor 6, a supplementary dampingof the circuits 2 and 3 occurs via the rectifiers 4 and 5. This resultsin the distortion of the demodulated signal at the terminal 9 beingreduced despite diflerent values of mean input amplitude, while theratio detector continues to present a satisfactory limiting eiiect.

Fig. 2 shows a known modification of the ratio detector of Fig. 1, therectifiers 4 and 5 having the same conductive direction and their outputincluding a detector filter 14 comprising two resistors 15 and 16 andtwo capacitors 17 and 18, the points of connection to which areconnected via the above described RC-filter 6, 7 which according to theinvention, has the VDR-resistor 10 connected in parallel therewith. Theother reference numerals designate like circuit elements as in Fig. 1.

What is claimed is:

A ratio detector for demodulating frequency-modulated oscillationscomprising first and second resonant circuits in coupled relationship,means connected to supply said oscillations to said first resonantcircuit, a pair of seriesconnected capacitors, a first diode having ananode connected to one end of said second resonant circuit and a cathodeconnected to one end of said series-connected capacitors, a second diodehaving an anode connected to the other end of said second resonantcircuit and a cathode connected to the other end of saidseries-connected capacitors, a pair of series-connected resistorsconnected in parallel with said pair of series-connected capacitors, acoil coupled to said first resonant circuit and connected at one end tosaid second resonant circuit, the other end of said coil being connectedto the junction between said pair of series-connected capacitors, aresistance-capacitance filter for by-passing modulation frequencies,said filter comprising a resistor and a capacitor connected in parallelcombination, said combination being connected at one end to saidcapacitor junction and at the other end to the junction between saidpair of series-connected resistors, said filter further including avoltage-responsive resistor the resistance of which decreases inresponse to increased applied voltage and increases in response todecreased applied voltage regardless of the polarity of the appliedvoltage, said last-named resistor being connected in parallel with saidparallel combination, and an output terminal coupled to the cathode ofsaid first diode.

References Cited in the file of this patent UNITED STATES PATENTS AmosOct. 23, 1951

